1. Field of the Invention
The present invention relates to a camera, and in particular, to a focus controller for focusing an imaging lens by use of a video signal attained from an imaging device.
2. Description of the Related Art
In conventional focus controllers, there has been employed a high frequency detection focus control method in which components of a video signal having a relatively high frequency are used such that the integrated value thereof develops the maximum value and effects the control to focus an imaging lens. Since the high frequency components are extracted from a horizontal scanning line of a video signal in this method. A characteristic of a satisfactory focusing precision is obtained for a pattern or design containing high freqeuncy components in the horizontal direction of the image, such as a pattern of vertical stripes.
In actual cases, there exist various objects ranging from objects having an obscure contour to objects having a clear contour. For a pattern with a clear contour, a large portion of a video signal attained by imaging the pattern is occupied by a high frequency component; and such a large high frequency component cannot be necessarily attained in a case of a pattern having an obscure contour. For a pattern of vertical stripes including clear white and black stripes shown as an example of the former case in FIG. 10A, when an imaging system is focused onto this object, there is attained a luminance signal Y of a rectangular shape as shown in FIG. 10B. When the luminance signal Y becomes similar to an ideal rectangular wave, the higher harmonic spectra thereof are increased. As a result, there is developed a frequency charateristic of FIG. 10D in which the frequency (f)-gain (g) characteristic is indicated by a solid line 400. When the focusing is not appropriately effected onto the object, the luminance signal Y becomes to be a waveform having a small harmonic component as shown in FIG. 10C. As a consequence, the frequency band thereof shifts toward a low frequency zone like a curve 402 of FIG. 10D. In this situation, although the fundamental or dominant wave greatly varies depending on the number of white and black stripes in the screen, if the video signals of the screen include even one impulse component, a focused state can be obtained in a relatively easy manner, for example, by use of a peak detection.
However, this conventional focus control method has a disadvantage that a high focusing accuracy cannot be developed for a pattern in which the luminance of the screen alters through an intermediate gradation. For example, in the case of an object as shown in FIG. 11A where a single light is irradiated onto a surface of a wall slightly having a wave-like contour along a horizontal direction H and there does not exist a reflective image due to a reflection, a luminance signal obtained from this object develops a waveform having a small harmonic wave component even in the focused state as shown in FIG. 11B. If this is an ideal sine or sinusoidal wave, an electric power spectra thereof naturally includes only the fundamental wave as shown in FIG. 11D, namely, the wave cannot be expected to obtain a high frequency component. In a state other than the focused state, as shown in FIG. 11C, only a slight frequency variation of the fundamental wave and a variation in the amount of the fundamental component appears.
In addition, the conventional focus control method has been attended with a disadvantage that a high focusing accuracy cannot be obtained also for a pattern in which the luminance is greately changed only in the vertical direction of the screen. In more detail of the video signal, a lower frequency, for example, a field frequency of 60 Hz is used for the scanning operation in the vertical direction. As a consequence, in the case where there exists only one line associated with the video signal in which a relatively large change takes place in the luminance along the vertical direction of the image, the change appears only in the fundamental wave of the field frequency so as to develop a component of a lowest frequency. Consequently, in a circuit above controlling the focusing operation by extracting the high frequency component, it has been impossible to effectively use such a low frequency component.
For example, as shown in FIG. 12A, in the case of a pattern of a horizontal design associated with an abrupt change in the gradation of the white and black along the vertical direction V, the high frequency component described above cannot be obtained in the video signal. This is because that the scanning is repeatedly achieved for the video signal in the horzontal direction H of the screen and hence the gradation change in the vertical direction V appears only in the dominant wave of the field frequency. As consequence, in the focused state, a focus detection signal is obtained as a luminance variation astriding two lines adjacent to each other as shown in FIG. 12B. In contrast, when the lens is not set to the focused state, as shown in FIG. 12C, the luminance variation gradually occurs through several horizontal scanning lines. Consequently, the frequency as an object of the focus control is completely different from the frequency in the case where the pattern including hrozontal stripes, and hence the focus control processing cannot be effected by use of the conventional focus controller. As described above, it has been impossible for the conventional focus controller to accomplish a sufficient focus control on an object associated with a luminance change in the vertical direction of the screen.